IValueConverter.Convert method

Applies to Windows and Windows Phone

Modifies the source data before passing it to the target for display in the UI.

Syntax

Function Convert(
  value As Object,  
  targetType As Type,  
  parameter As Object,  
  language As String 
) As Object

Parameters

value

Type: System.Object [.NET] | Platform::Object [C++]

The source data being passed to the target.

targetType

Type: System.Type [.NET] | TypeName [C++]

The type of the target property, as a type reference (System.Type for .NET, a TypeName helper struct for C++/CX).

parameter

Type: System.Object [.NET] | Platform::Object [C++]

An optional parameter to be used in the converter logic.

language

Type: System.String [.NET] | Platform::String [C++]

The language of the conversion.

Return value

Type: System.Object [.NET] | Platform::Object [C++]

The value to be passed to the target dependency property.

Remarks

The targetType parameter of the Convert method uses different techniques of reporting the type system info, depending on whether you're programming with .NET or C++/CX.

  • For .NET, this parameter passes an instance of the System.Type type.
  • For C++/CX, this parameter passes a TypeName structure value. TypeName::Kind contains the simple string name of the type, similar to .NET's Type.Name.

When the converter is invoked by the binding engine, the targetType value is passed by looking up the property type of the target dependency property. You might use this value in your Convert implementation for one of two reasons:

  • Your converter has the expectation that it's always going to return objects of a specific type, and you want to verify that the binding that the converter is called for is using the converter correctly. If not, you might return a fallback value, or throw an exception (but see "Exceptions from converters" below).
  • Your converter can return more than one type, and you want the usage to inform your converter which type it should return. For example, you could implement an object-to-object conversion and an object-to-string conversion within the same converter code.

language comes from the ConverterLanguage value of a specific binding, not system values, so you should expect that it might be an empty string.

parameter comes from the ConverterParameter value of a specific binding, and is null by default. If your converter uses parameters to modify what it returns, this usually requires some convention for validating what is passed by the binding and handled by the converter. A common convention is to pass strings that name modes for your converter that result in different return values. For example you might have "Simple" and "Verbose" modes that return different length strings that are each appropriate for display in different UI control types and layouts.

Exceptions from converters

The data binding engine does not catch exceptions that are thrown by a user-supplied converter. Any exception that is thrown by the Convert method, or any uncaught exceptions that are thrown by methods that the Convert method calls, are treated as run-time errors. If you are using the converter in situations where the binding can use fallbacks or otherwise show reasonable results even if a conversion failure occurs, consider having your converter return DependencyProperty.UnsetValue and not throw exceptions. DependencyProperty.UnsetValue is a sentinel value that has special meaning in the dependency property system, and bindings that are passed this value will use FallbackValue.

Another alternative to throwing exceptions is to return the original value unchanged, and let the binding instance handle what it might do with that value. In most cases UI bindings that fail won't be error cases. They just won't use the source value and will instead use DependencyProperty.UnsetValue to show nothing, or use fallbacks.

try/catch based on doing something to value is a common implementation pattern for the Convert method, but you shouldn't rethrow, for the reasons mentioned above.

Examples

The following example shows how to implement the Convert method using the parameter and language parameters.

Imports System.Collections.ObjectModel
Imports System.Windows.Data
Imports System.Globalization

Partial Public Class Page
    Inherits UserControl

    Public MyMusic As New ObservableCollection(Of Recording)()
    Public Sub New()
        InitializeComponent()

        ' Add items to the collection.
        MyMusic.Add(New Recording("Sheryl Crow", "Detours", New DateTime(2008, 2, 5)))
        MyMusic.Add(New Recording("Brandi Carlisle", "The Story", New DateTime(2007, 4, 3)))
        MyMusic.Add(New Recording("Patty Griffin", "Children Running Through", New DateTime(2007, 2, 6)))

        ' Set the data context for the combo box.
        MusicCombo.DataContext = MyMusic
    End Sub
End Class

' Simple business object. 
Public Class Recording
    Public Sub New()
    End Sub
    Public Sub New(ByVal artistName As String, ByVal cdName As String, _
       ByVal release As DateTime)
        Artist = artistName
        Name = cdName
        ReleaseDate = release
    End Sub
    Private artistValue As String
    Private nameValue As String
    Private releaseDateValue As DateTime
    Public Property Artist() As String
        Get
            Return artistValue
        End Get
        Set(ByVal value As String)
            artistValue = value
        End Set
    End Property
    Public Property Name() As String
        Get
            Return nameValue
        End Get
        Set(ByVal value As String)
            nameValue = value
        End Set
    End Property
    Public Property ReleaseDate() As DateTime
        Get
            Return releaseDateValue
        End Get
        Set(ByVal value As DateTime)
            releaseDateValue = value
        End Set
    End Property
End Class

Public Class DateFormatter
    Implements IValueConverter

    ' This converts the DateTime object to the string to display. 
    Public Function Convert(ByVal value As Object, ByVal targetType As Type, _
        ByVal parameter As Object, ByVal language As System.String) As Object _
        Implements IValueConverter.Convert

        ' Retrieve the format string and use it to format the value. 
        Dim formatString As String = TryCast(parameter, String)
        If Not String.IsNullOrEmpty(formatString) Then

            Return String.Format(New CultureInfo(language), formatString, value)
        End If

        ' If the format string is null or empty, simply call ToString() 
        ' on the value. 
        Return value.ToString()
    End Function

    ' No need to implement converting back on a one-way binding.
    Public Function ConvertBack(ByVal value As Object, ByVal targetType As Type, _
        ByVal parameter As Object, _
        ByVal language As System.String) As Object _
        Implements IValueConverter.ConvertBack
        Throw New NotImplementedException()
    End Function
End Class

Requirements

Minimum supported client

Windows 8

Minimum supported server

Windows Server 2012

Minimum supported phone

Windows Phone 8.1 [Windows Runtime apps only]

Namespace

Windows.UI.Xaml.Data
Windows::UI::Xaml::Data [C++]

Metadata

Windows.winmd

See also

IValueConverter
ConvertBack
XAML data binding sample
Data binding overview

 

 

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